如何在 Python 中使用 C 风格的 for 循环? [英] How do I use a C-style for loop in Python?

问题描述

我想使用传统的 C 风格的 for 循环Python.我想遍历字符串的字符，但也知道它是什么，并且能够跳过字符(例如 i =5 代码中的某处).

I want to use the traditional C-style for loop in Python. I want to loop through characters of a string, but also know what it is, and be able to jump through characters (e.g. i =5 somewhere in the code).

for with range 没有给我实际 for 循环的灵活性.

for with range doesn't give me the flexibility of an actual for loop.

推荐答案

简单的答案是，Python 中没有简单、精确的 C 语言 for 语句的等价物.使用带有范围的 Python for 语句涵盖的其他答案.如果您希望能够在循环中修改循环变量(并使其影响后续迭代)，则必须使用 while 循环:

The simple answer is that there is no simple, precise equivalent of C's for statement in Python. Other answers covered using a Python for statement with a range. If you want to be able to modify the loop variable in the loop (and have it affect subsequent iterations), you have to use a while loop:

i = 0
while i < 7:
    if someCondition(i):
        i = 5
    i += 1

但在该循环中，continue 语句的效果与 C for 循环中的 continue 语句的效果不同.如果你想让 continue 像在 C 中那样工作，你必须加入一个 try/finally 语句:

But in that loop, a continue statement will not have the same effect that a continue statement would have in a C for loop. If you want continue to work the way it does in C, you have to throw in a try/finally statement:

i = 0
while i < 7:
    try:
        if someCondition(i):
            i = 5
        elif otherCondition(i):
            continue
        print 'i = %d' % i
    finally:
        i += 1

如您所见，这非常难看.您应该寻找一种更 Pythonic 的方式来编写您的循环.

As you can see, this is pretty ugly. You should look for a more Pythonic way to write your loop.

这刚刚发生在我身上......有一个复杂的答案，它可以让您像 C 样式循环一样使用普通的 Python for 循环，并允许通过编写自定义来更新循环变量迭代器.我不会将这个解决方案推荐给任何真正的程序，但它是一个有趣的练习.

This just occurred to me... there is a complicated answer that lets you use a normal Python for loop like a C-style loop, and allows updating the loop variable, by writing a custom iterator. I wouldn't recommend this solution for any real programs, but it's a fun exercise.

C 风格"for 循环示例:

Example "C-style" for loop:

for i in forrange(10):
    print(i)
    if i == 3:
        i.update(7)

输出:

0
1
2
3
8
9

诀窍是 forrange 使用了 int 的子类，它添加了一个 update 方法.forrange 的实现:

The trick is forrange uses a subclass of int that adds an update method. Implementation of forrange:

class forrange:

    def __init__(self, startOrStop, stop=None, step=1):
        if step == 0:
            raise ValueError('forrange step argument must not be zero')
        if not isinstance(startOrStop, int):
            raise TypeError('forrange startOrStop argument must be an int')
        if stop is not None and not isinstance(stop, int):
            raise TypeError('forrange stop argument must be an int')

        if stop is None:
            self.start = 0
            self.stop = startOrStop
            self.step = step
        else:
            self.start = startOrStop
            self.stop = stop
            self.step = step

    def __iter__(self):
        return self.foriterator(self.start, self.stop, self.step)

    class foriterator:

        def __init__(self, start, stop, step):
            self.currentValue = None
            self.nextValue = start
            self.stop = stop
            self.step = step

        def __iter__(self): return self

        def next(self):
            if self.step > 0 and self.nextValue >= self.stop:
                raise StopIteration
            if self.step < 0 and self.nextValue <= self.stop:
                raise StopIteration
            self.currentValue = forrange.forvalue(self.nextValue, self)
            self.nextValue += self.step
            return self.currentValue

    class forvalue(int):
        def __new__(cls, value, iterator):
            value = super(forrange.forvalue, cls).__new__(cls, value)
            value.iterator = iterator
            return value

        def update(self, value):
            if not isinstance(self, int):
                raise TypeError('forvalue.update value must be an int')
            if self == self.iterator.currentValue:
                self.iterator.nextValue = value + self.iterator.step

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